It is well known that the level flight power required for helicopters to hover decreases as air speed is increased above 0 knots. In the absence of some relative wind, hovering requires more power than slow speed forward flight. Most helicopters experience a large change in power required for level flight in the speed range of from 10 to 40 knots. As the air speed increases to 40 knots and somewhat beyond, the power required for level flight is reduced.
To reduce speeds of a helicopter from speeds above 40 knots to relative air speeds of about 10 knots, a great increase in power is required. A further increase in power required, although usually not so much, is necessary for hovering when the air speed is reduced from 10 knots to 0 knots.
As speeds increase from about 60 knots upward, the power required for level flight increases, but usually at a lower rate than the rate of power increase required to slow from about 40 knots to 10 knots.
The power required for level flight is a function of air speed and a ratio of gross weight of the heilcopter to density ratio of the air compared with the density at sea level. For relatively large gross weights steep power increases are required. For smaller gross weights increases of power to slow the helicopter and to hover are less steep.
Each helicopter model is usually accompanied by graphs which compare pressure altitudes or density altitudes with gross weights and head winds and which plot several different curves for different ambient temperatures. Determining expected operational parameters of ambient temperatures and pressure altitudes, pilots use the charts to determine the gross weight which they will be able to carry. When continous winds are expected, the pilot can reference the charts to determine the additional weight which can be carried during takeoff or landing.
The charts are cumbersome and are difficult and time consuming to use. The determinations of estimated pressure, altitude and ambient temperature, necessary to enter on the charts, are difficult to make accurately. The gross weight of a craft is difficult to ascertain and to limit accurately. For various reasons such as operational hours, maintenance conditions, fuel variations and ambient conditions, power output capabilities of a craft may not be known.
All these factors lead to the making of conservative estimations on charts. Pilots, recognizing the conservative data, may be prone to exceed the charged limitations.
Present day helicopters are equipped with power measuring and displaying apparatus and particularly apparatus to measure reaction torque between an engine and a main transmission. The reaction is presented in the cockpit in terms of normal rated power or take-off power. In some cases, torque is presented in terms of pressure, for example, from 0 to 60 pounds per square inch. Engine torque display is used by pilots to set and adjust power demand.
Pilot flight manuals provide information showing how power changes with increasing wind and how maximum take-off gross weight changes with wind. The intent of the information is the same in both cases. In both cases a density altitude at operating conditions is selected. The pilot next determines the power or weight trade-off with changing air speed.
Prudent helicopter pilots prefer to operate with a known or high maneuver margin capability to provide available reaction power for unforeseen circumstances. However, because of the steep increase of power requirement, particularly for helicopters with high gross weights, the actual maneuver margin capability of a helicopter is hard to estimate as the aircraft slows to a hover.
Complex apparatus and systems have been devised with the object of determining helicopter lift margins. Heretofore, no system has been provided to present in a simple manner maneuver margins for helicopters.